The purpose of this project is the collaborative study of the physical properties of a wide variety of biological macromolecules with the goal of correlating these properties with the structure and function of the macromolecules. Analytical ultracentrifugation and mathematical modeling are the principal research techniques used. An area of major emphasis has been collaborative studies with the laboratory of Dr. Samuel Wilson (NIEHS) on proteins involved in DNA transcription initiation and in DNA repair. Studies in progress are (1) the interactions between DNA Ligase I and the replication protein, proliferating cell nuclear antigen (PCNA); (2) the interaction mechanisms between the XRCC1(1-183) protein and DNA polymerase-beta and its subdomains; (3) the study of DNA transcription initiation repression by gal repressor (galR) and the HU protein; and (4) the interactions between AP endonuclease and DNA, DNA polymerase-beta and DNA, and both together with DNA. Studies on the associative behavior of translin, a protein involved in translocation of chromosomal DNA, have been done in a collaboration with Dr. Myun Ki Han (Georgetown University) and Dr. Jay Knutson (NHLBI). New studies on translin clearly demonstrated that the prior studies which suggested that translin undergoes a monomer-octamer reversible association were in error. Translin octamer was isolated and it was definitively established that the octamer did not dissociate and that the octameric form bound single-stranded DNA. These studies utilized the newly developed technique of multi-wavelength analysis where one or more of the interacting components has an added chromophore label that permits observing the specific behavior of the labeled component. (See Project 1Z01 OD10039-06, "Biophysical Instrumentation and Methodology.") In addition, a collaborative investigation of the thermodynamics of the self-association of HIV-1 integrase and the effects of DNA binding on this assocation are also under way with Drs. Han, Knutson and John Harvey (NHLBI), also using this new technology. New studies on the tetramerization domain of p53 have been initiated. The peptide studied comprises the central core domain which is responsible for the antiparallel dimer of dimers structure and extends just past the first of the potentially phosphorylatable serine residues. We have investigated the effect of phosphorylation of this serine on the thermodynamics of tetramerization and have found that while there is relatively little difference in the free energy changes of association at lower temperatures, there is a modest but significant increase in the magnitude of the free energy changes as the temperature is increased and that this is caused by an increase of the entropy change with phosphorylation. Charge repulsion forces between the phosphoryl groups can be postulated as a possible cause for this entropic effect.